Comparison of Compact Liquid Crystal-based Phase Shifting Topologies for
Reconfigurable Intelligent Surfaces
Abstract
This paper presents and compares two promising approaches for realizing
liquid crystal (LC)-based phase shifting topologies for Reconfigurable
Intelligent Surfaces (RIS). Main requirements for the phase shifters
include high Figure of Merit (FoM), compact size, low response times and
constant group delay. The first approach is based on defective ground
structure inverted microstrip lines (DGS-IMSL) and shows high FoM above
74 °/dB between 25 and 30 GHz. In addition, it promises low response
times due to its thin 4.6 µm LC layer and features moderate physical
length below 0.45 λ0. However, it is limited by a non-constant group
delay. The second approach, the bandpass inverted microstrip line
(BP-IMSL), has a lower FoM of approximately 26 °/dB between 28 to 30 GHz
with a thicker LC layer of 20 µm. Nevertheless, a shorter physical size
below 0.25 λ0 is accomplished and potential for a flat group delay is
given.